A radar system transmits a signal and receives its echo. By processing the echo signal, the radar system is able to detect objects, and to estimate the distances, velocities, and directions associated with the objects. Historically, a pulsed radar is used in military applications, where targets of interest are typically far away from the radar system. The pulsed radar emits short pulses, and in the silent period receives the echo signals. The transmitter of the pulsed radar system is turned off before the measurement starts. However, in many civilian applications, such as automotive radar, wireless gesture recognition, vital sign monitoring, and other monitoring implementations, the objects of interest are usually close to the radar. Due to the short round-trip-delay (RTD) of the desired reflection signal, a pulsed radar doesn't work as well at close range. Instead of a pulsed radar, a frequency-modulated continuous wave or waveform (FMCW) radar is used for short distances.
In FMCW radar, the transmission signal is modulated in frequency (or in phase) and differences in phase or frequency between the transmitted signal and a received signal are used to measure distance to the object from which the transmitted signal is reflected. A linear frequency modulated (LFM) waveform can be used, whose instantaneous frequency linearly increases or decreases over time. With the change in frequency being linear over a wide range, then the distance can be determined by a frequency comparison, with the frequency difference being proportional to the distance. However, in practice, nonlinearity exists in the frequency sweep of the transmitted waveform. This can result in severe performance degradation.